This invention relates to the production of aluminum oxide. More particularly, this invention relates to the production, from a Bayer process, of aluminum oxide having low sodium oxide content.
Normally, in the production of aluminum oxide from a Bayer process, economic considerations dictate precipitation of the aluminum oxide (in the hydroxide form) from the mother liquor at the fastest possible rate in order to minimize tank volume required to produce a given yield. At any given temperature the degree of supersaturation drives the precipitation rate with the fastest rate being accomplished at higher ratios of aluminum oxide to sodium oxide. Unfortunately, precipitation of aluminum hydroxide from the liquor at a high aluminum oxide to sodium oxide ratio results in a precipitate which is high in sodium oxide content. By "high" is meant greater than 0.2% by weight Na.sub.2 O in the Al.sub.2 O.sub.3.
While it is known to precipitate aluminum hydroxide at a slow rate to achieve a precipitate having a low sodium oxide content, this is normally accomplished using temperature control by raising the precipitation tank to an elevated temperature thus lowering the degree of saturation resulting in a lower precipitation rate. A considerable amount of energy, however, must be expended in providing such an elevated temperature to a large tank. It thus would be desirable to achieve such controlled precipitation at a low aluminum oxide to sodium oxide ratio without the need for such elevated temperatures.